During my PhD I worked mostly on passively mode-locked c.w. femtosecond dye lasers, under the supervision of Wilson Sibbett, Roy Taylor and Geoff New. Working with Roy Taylor, I demonstrated that such lasers could provide broad spectral coverage spanning from the blue to the near infrared. I built the first sub-100 femtosecond (fs) dye laser in the UK and demonstrated the world's first colliding pulse passively mode-locked (CPM) ring dye laser to use a dye combination other than Rhodamine 6G and DODCI. I published a theoretical paper on intracavity dispersion compensation using Gires-Tournois Interferometers and also did some work on synchronous pumping of dye lasers and pulse propagation in optical fibres. As a PDRA I developed the world's first femtosecond c.w. passively mode-locked blue laser.
As an SERC Post-Doctoral Research Fellow, I became nominally independent and worked in Roy Taylor's laboratory extending the palette of femtosecond dye lasers, including systems operating in the blue-green and near infra-red. I demonstrated the first blue CPM laser, which could be used at 497 nm to provide seed pulses for a KrF excimer laser amplifier. Working with Hercules Avramopoulos and Geoff New, I developed a comprehensive experimental and theoretical analysis of a femtosecond dye laser. With Roy Taylor, I worked on ultrafast solid-state lasers, building the first c.w. Ti:sapphire laser in the UK, and demonstrated the first subpicosecond Ti:sapphire laser in the world, using a nonlinear (fibre) external cavity with an acousto-optic modulator to initiate mode-locking. (At almost the same time an independent and parallel programme at MIT demonstrated self-starting mode-locking of a Ti:sapphire laser with this mechanism, described as Additive Pulse Mode-locking or Coupled Cavity Mode-locking.)
As a Royal Society University Research Fellow I continued to work with Roy Taylor, studying femtosecond dye lasers and developing new solid-state lasers. I discovered the technique of "moving mirror mode-locking", a variation of which is used to initiate Kerr Lens Mode-locking (KLM) in some commercial state-of-the-art ultrafast lasers today. I proposed a novel scheme for pedestal suppression based on second harmonic generation. In 1990/1 I worked at AT&T Bell laboratories, Holmdel, NJ., in Alan Huang's Department, where I worked on all optical switching used nonlinear fibre Sagnac interferometers. I provided ultrafast laser expertise to the team that demonstrated arbitrary all-optical switching and demultiplexing for the first time. After returning to the UK, I continued working with Roy Taylor and jointly supervised PhD students working on ultrafast solid-state lasers. Initially we investigated mode-locked Ti:sapphire lasers and demonstrated moving mirror mode-locking to initiate Kerr Lens Mode-locking. Subsequently we started our Cr:LiSAF laser programme with the intention of ultimately developing diode-pumped tunable ultrafast lasers. Using 488 nm argon ion excitation we generated the first sub-100 fs pulses from Cr:LiSAF, achieving 33 fs, which was a record for many years. Later we demonstrated the mode-locking Cr:LiSAF and Ti:sapphire lasers using intracavity MQW saturable absorbers (to initiate KLM) for the first time. This was significant since it was widely considered to be impossible for such low gain lasers to sustain the insertion losses of such absorbers. We then built the first all-solid-state diode-pumped tunable lasers, initially exploiting active mode-locking and then passive mode-locking with intracavity MQW saturable absorbers. In this way we demonstrated the first diode-pumped femtosecond (Cr:LiSAF) vibronic laser. We went on to examine pulse evolution dynamics in such lasers and also demonstrated the world's first diode-pumped tunable solid-state regenerative amplifier, based on Cr:LiSAF. In a parallel programme I co-supervised students working on other solid-state laser systems. We demonstrated the first ultrafast c.w. mode-locked Cr:YAG laser and the first sub-100 fs Cr:YAG laser. We also measured the intracavity GVD of Cr:YAG lasers using a technique I adapted from one developed at Bell Laboratories. We also demonstrated the first c.w. ultrafast visible solid-state laser, using KLM to mode-lock Pr:YLF. Other work in this period included a diode-pumped tunable microchip laser, based on Cr:LiSAF.
As a member of academic staff at Imperial College I continued my joint research with Roy Taylor on solid-state lasers and started my programme on the application of ultrafast lasers to biomedical imaging. The former continued with the development of the all-solid-state tunable regenerative oscillators and amplifiers, based on diode-pumped Cr:LiSAF; we also built a novel argon ion laser-pumped oscillator-amplifier system. Working with c.w. Pr:YLF lasers, we demonstrated the use of a solid-state semiconductor-doped glass saturable absorber to initiate KLM and later lased 14 new transitions in this medium and built the first femtosecond c.w. visible solid-state laser. We also worked on Cr:YAG and Cr:Forsterite femtosecond lasers and studied self-starting KLM in several systems. My group started to apply this ultrafast laser technology including to ultrafast all-optical storage in collaboration with Carmen Afonso and Javier Solis' group from CSIC, Spain. Our other principal applications, for which Chris Dainty was initially a co-investigator, were part of my biomedical imaging programme and included 3-D imaging through turbid media using photorefractive holography and fluorescence lifetime imaging (FLIM). The former work concerned my (patented) novel real-time 3-D imaging system based on photorefractive holography. We demonstrated the 3-D imaging through turbid media with unprecedented whole-field resolution and improved this further to achieve real-time 3-D imaging with sub-100 micron resolution and application through turbid media. Subsequently, in collaboration with David Nolte's group at Purdue University, we incorporated MQW photorefractive devices as the holographic recording medium to produce the fastest whole-field 3-D imaging system ever demonstrated. Our fluorescence lifetime imaging system exploits our home-grown ultrafast amplifier technology in conjunction with wide-field time-gated image intensifier technology developed by Kentech Instruments Ltd and previously demonstrated by David Phillips group. We developed the first wide-field time-domain FLIM system to be integrated with an ultrafast solid-state laser system and demonstrated the best wide-field temporal resolution yet reported.
Recently most of my group's research has been primarily concerned with developing biomedical imaging instrumentation, although I have continued some activity on ultrafast lasers. The latter programme has produced further refinements of our diode-pumped all-solid-state fs Cr:LiSAF technology and we have demonstrated some all-solid-state diode-pumped ultrafast Cr4+ lasers. With Geoff New I have jointly supervised PhD students working on models of KLM in order to develop optimised diode-pumped KLM lasers. This model is used to optimise our KLM Cr:LiSAF lasers and we continue to develop robust diode-pumped ultrafast laser technology, which are applying to biomedical imaging.
My 3-D holographic imaging programme has progressed with the demonstration of the high background noise suppression of photorefractive holography and the demonstration of real-time "direct to video" read out of the depth-resolved images. This high-speed imaging is possible due to the underlying physics of the photorefractive MQW devices and can potentially realise depth-resolved frame rates exceeding 1000/second. One of the most significant challenge remaining for this technique to find utility in biomedical imaging is the problem of speckle that is common to all wide-field coherent imaging modalities. We have begun investigating the use of spatially incoherent sources and have demonstrated 3-D holographic imaging using LED's, multi-mode diode lasers and high power diode arrays. We are currently applying the instrument to microscopy and imaging through tissue. We have also developed a novel broadband diode-pumped c.w. laser that exploits spectral dispersion in the gain medium. With IC Innovations Ltd, I have started a company called Holoscan UK Ltd to commercially develop the technique of holographic imaging using photorefractive holography.
Our fluorescence lifetime imaging programme has expanded into two main areas: non-invasive functional imaging of biological tissue and functional imaging of cell biology. With John Lever we have studied the time-resolved autofluorescence of unstained tissue in vitro, demonstrating strong intrinsic lifetime contrast between collagen and elastin and between different states of elastin. We have also demonstrated the ability of our wide-field FLIM instrument to image the local fluorophore environment and, using viscosity, have demonstrated an unprecedented temporal discrimination permitting us to image lifetime differences of less than 10 ps. My group has recently demonstrated our FLIM instrument in microscope geometry with an all-solid-state diode-pumped laser system and with a pulsed blue diode laser. This are significant world firsts that indicate great potential for future practical instrumentation. With the groups of John Lever, Andrew Wallace, Gordon Stamp and Charles Coombs, we are currently applying this technology to the detection of cancer and are developing a wide-field in vivo FLIM endoscope system. Our functional imaging programme is developing in collaboration with the groups of Dan Davis and David Phillips and has been applied to imaging intracellular proteins labelled with EGFP. We are using FLIM to probe the local fluorophore environment and have demonstrated that the fluorescence lifetime of EGFP is sensitive to the local solvent refractive index. We have also developed a time-resolved polarisation anisotropy imaging system that yield maps of the "true" fluorescence lifetime and the rotational correlation time, which can be used to report on variations in solvent viscosity and binding of labelled proteins. With Tony Wilson's group from Oxford we have developed a wide-field optically sectioning fluorescence lifetime microscope. We have combined this with multispectral imaging to produce a wide-field 5-D fluorescence microscope that simultaneously resolves x, y, z, lifetime and wavelength.